Description

The Aggregation Module confers onto our bacteria the means of plastic adhesion. To implement this we have decided to transform our bacteria with a genetic circuit to produce adhesive proteins called Curlis. As curlis are non-specific in the surfaces they bind to, the Detection module will limit their production, unless they are in the presence of plastics.

Curli formation involves a gene cluster under the control of an operon promoter. The curli generating BioBrick we will be using (BBa_K540000), carries a cobalt promoter, which we will switch for the pSal promoter (BBa_K228004) described in the Detection module. Hence, curlis will encourage the adhesion of bacteria to the plastic fragment, and assist in the formation of biofilms. Adhesion between biofilm-covered plastic fragments will allow smaller plastic fragments to aggregate into larger plastic formations.

How are curlis produced? Curli production is complex, and not fully explained. It is known that there are two key structural proteins, CsgA and CsgB, which are reasonably well characterised. CsgA is the main structural component, which is a secreted from the cell as subunits. CsgB is also secreted, and is essential to allow the CsgA subunits to polymerise into fibrils. This is under the control of the CsgAB operon.

A second operon CsgDEFG, containing four regulatory genes, is also required for Curli production. The first of these, a transcriptional regulator called CsgD drives transcription of the genes of the CsgAB operon. Less is known of CsgE and CsgF of the CsgDEFG operon, but they appear to regulate or stabilise the fourth member of the operon – CsgG.
CsgG is an outer membrane lipoprotein, which interacts with itself to form an oligomeric ring-shaped structure through the membrane. This acts as a pore that allow CsgA and CsgB to leave the cell to form fibrils.

All of these constituents will have to be transformed into our bacteria, but while bacteria with a curli gene cluster naturally regulate curli production to respond to their environment, our system will place the production of curli under the control of the Detection circuit.